Vladimir G. Kozlov

Terahertz-wave generation in quasi-phase-matched GaAs

The authors demonstrate an efficient room temperature source of terahertz radiation using femtosecond laser pulses as a pump and GaAs structures with periodically inverted crystalline orientation, such as diffusion-bonded stacked GaAs and epitaxially grown orientation-patterned GaAs, as a nonlinear optical medium. By changing the GaAs orientation-reversal period (504–1277μm), or the pump wavelength (2–4.4μm), we were able to generate narrow-bandwidth (∼100GHz) terahertz wave packets, tunable between 0.9 and 3THz, with the optical-to-terahertz photon conversion efficiency of 3.3%.

Terahertz Sources Based on Intracavity Parametric Down-Conversion in Quasi-Phase-Matched Gallium Arsenide

We have efficiently generated tunable terahertz (THz) radiation using intracavity parametric down-conversion in gallium arsenide (GaAs). We used three types of microstructured GaAs to quasi-phase-match the interaction: optically contacted, orientation-patterned, and diffusion-bonded GaAs. The GaAs was placed in an optical parametric oscillator (OPO) cavity, and the THz wave was generated by difference-frequency mixing between the OPO signal and idler waves. The OPO used type-II phase-matched periodically poled lithium niobate as a gain medium and was synchronously pumped by a mode-locked laser at 1064 nm (7 ps and 200 nJ at 50 MHz). With center frequencies spanning 0.4-3.5 THz, 250-GHz bandwidth radiation was generated. We measured two orders of optical cascading generated by the mixing of optical and THz waves. In a doubly resonant oscillator (DRO) configuration, the efficiency increased by 21times over the singly resonant oscillator performance with an optical-to-THz efficiency of 10-4 and average THz power of 1 mW. The GaAs stabilized the DRO by a thermooptic feedback mechanism that created a quasi- continuous-wave train of THz pulses.

Generation of multicycle terahertz pulses via optical rectification in periodically inverted GaAs structures

The authors demonstrate the generation of multicycle narrow-bandwidth terahertz pulses in periodically inverted GaAs structures using optical rectification of 2μm, 100fs pump pulses. Three different types of orientation-inverted samples are employed: optically contacted multilayer, orientation-patterned, and diffusion-bonded GaAs. The terahertz pulses are characterized by two-color (pump at 2μm and probe at 0.8μm) terahertz time-domain spectroscopy and terahertz Michelson interferometry.

Photonic THz generation in GaAs via resonantly enhanced intracavity multispectral mixing

We generate tunable (1.5–2 THz) terahertz output with up to 200 μW average power in periodically inverted GaAs using resonantly enhanced multispectral frequency mixing inside the cavity of a type-0 optical parametric oscillator operating at degeneracy. The optical parametric oscillator was synchronously pumped by a 1064-nm picosecond Yb-fiber laser and produced, due to the presence of an intracavity Fabry-Perot etalon, a set of optical frequency peaks spaced at the desired THz interval that allows efficient THz wave production via difference frequency generation. The proposed method is well adapted for cascaded THz generation, where the quantum conversion limit can be significantly surpassed.

Terahertz wave generation in orientation-patterned GaAs using resonantly enhanced schemes

Zincblende semiconductors (GaAs, GaP) show great potential for quasi-phase-matched (QPM) THz generation because of their small (20 times less than in lithium niobate) absorption coefficient at terahertz frequencies, small mismatch between the optical group and THz phase velocities, high thermal conductivity, and decent electro-optical coefficient. Terahertz-wave generation was demonstrated recently in QPM GaAs, using optical rectification of femtosecond pulses. Here we report on a new efficient widely tunable (0.5-3.5 THz) source of THz radiation based on quasi-phase-matched GaAs crystal. The source is based on difference frequency generation inside the cavity of a synchronously pumped near-degenerate picosecond OPO and takes advantage of resonantly enhanced both the signal and the idler waves. THz average power as high as 1 mW was achieved in a compact setup.

Terahertz-wave generation in periodically-inverted GaAs

Efficient generation of narrow-band terahertz-wave pulses tunable in the 2-3 THz frequency range was achieved in periodically-inverted GaAs; structures pumped by 100-fs pulses tunable between 2 and 4 microns, via mechanism of quasi-phase-matched optical rectification.

THz-wave generation inside a high-finesse ring-cavity OPO pumped by a fiber laser

Substantial improvement in the efficiency of photonic THz-wave generation via frequency downconversion results from resonant cavity enhancement. Previously, efficient THz wave generation was demonstrated at 2.8 THz by difference frequency mixing between resonating signal and idler waves of the linear-cavity type-II-phase-matched PPLN optical parametric oscillator (OPO). We present a new, simplified approach to resonantly-enhanced THz-wave generation in periodic GaAs, featuring (i) ring, instead of linear, OPO cavity with much higher finesse, (ii) type-0, instead of type-IIphase- matched PPLN crystal as a gain medium, resulting in much lower OPO threshold, (iii) a compact picosecond 1064-nm fiber laser as a pump source, and (iv) the use of a thin intracavity etalon with a free spectral range equal to the desired THz output frequency. 2.1 μm anti-reflection coated stacks of optically contacted GaAs wafers (OC-GaAs) and diffusion bonded GaAs wafers (DB-GaAs) with periodic-inversion were placed in the second OPO focal plane for intracavity THz generation. Narrowband output in the range 1.4 - 3 THz was produced with more than 130 microwatts of average power at 1.5 THz using 6.6 W of average pump power. The demonstrated approach can be extended to generate 1-10 mW of THz output in a compact setup by optimizing the OPO PPLN crystal length and optimizing spectral characteristics of the fiber pump laser and OPO.

Generation of multi-cycle THz-pulses via optical rectification in periodically inverted GaAs

We demonstrate an efficient room temperature source of narrow-bandwidth terahertz (THz) radiation using femtosecond pump pulses and periodic GaAs structure as a nonlinear material. In the past, several THz generation schemes exploited optical rectification in nonlinear crystals using femtosecond laser technology. Most of them generated single-cycle THz-pulses with broad bandwidth, using nonlinear crystals shorter than the phase-matching coherence length. Recently a novel technique to generate multi-cycle THz-pulses in the pre-engineered domain structure of periodically-poled lithium niobate (PPLN) crystals has been demonstrated. Quasi-phase matching (QPM) structures such as PPLN consist of a periodic system of domains of inverted crystal orientation. The sign of second order nonlinear polarization generated by femtosecond pulses is inverted at domain boundaries. If domain length is comparable with coherence length, QPM between THz-wave and nonlinear polarization extends the interaction length between THz and optical pulses. In the present work, using periodic GaAs structures we have achieved exceptionally high photon as well as energy conversion efficiency: 3% and 0.07% respectively. We have examined two different types of periodic QPM GaAs samples: diffusion-bonded GaAs wafers and all-epitaxially-grown orientation-patterned GaAs crystals with 3-10 mm thicknesses. The incident optical pulse energy was in the micro-Joule range and pulse duration was ~100 fsec. We measured spectral properties of THz radiation using Michelson interferometer and a bolometer. Narrow-bandwidth (~100GHz) THz output, tunable between 1 and 3 THz, was achieved. THz frequency was tuned either by tuning the light source wavelength between 2 and 4.4 microns, or by selecting GaAs samples with different QPM periods. Our theoretical analysis, based on known GaAs dispersion properties, shows good agreement between the measured and predicted THz frequencies.

High power THz sources for nonlinear imaging

Many biological and chemical compounds have unique absorption features in the THz (0.1 - 10 THz) region, making the use of THz waves attractive for imaging in defense, security, biomedical imaging, and monitoring of industrial processes. Unlike optical radiation, THz frequencies can pass through many substances such as paper, clothing, ceramic, etc. with little attenuation. The use of currently available THz systems is limited by lack of highpower, sources as well as sensitive detectors and detector arrays operating at room temperature. Here we present a novel, high power THz source based on intracavity downconverison of optical pulses. The source delivers 6 ps pulses at 1.5 THz, with an average power of >300 μW and peak powers >450 mW. We propose an imaging method based on frequency upconverison that is ideally suited to use the narrow bandwidth and high peak powers produced by the source. By upconverting the THz image to the infrared, commercially available detectors can be used for real time imaging.

Intracavity terahertz generation from gallium arsenide in a fiber laser pumped type 0 doubly resonant optical parametric oscillator

Resonant cavity enhancement results in substantial improvement in the efficiency of photonic THz-wave generation via frequency down conversion. Efficient THz wave generation was demonstrated at 2.8 THz previously by difference frequency mixing between resonating signal and idler waves of the linear-cavity type-II-phase-matched PPLN optical parametric oscillator (OPO). A new, simplified approach to resonantly-enhanced THz-wave generation in periodic GaAs, featuring (i) ring, instead of linear, OPO cavity with much higher finesse, (ii) type-0, instead of type-II-phase-matched PPLN crystal as a gain medium, resulting in much lower OPO threshold, (iii) a compact picosecond 1064-nm fiber laser as a pump source, and (iv) the use of a thin intracavity etalon with a free spectral range equal to the desired THz output frequency is presented here. Intra-cavity THz generation was performed by 2.1 μm anti-reflection coated stacks of optically contacted GaAs wafers (OC-GaAs) and diffusion bonded GaAs wafers (DB-GaAs) with periodic-inversion placed in the second OPO focal plane. Using 6.6 W of average pump power, narrowband output in the range 1.4 - 3 THz was produced with more than 130 microwatts of average power at 1.5 THz. By optimizing the OPO PPLN crystal length and spectral characteristics of the fiber pump laser and OPO the demonstrated approach can be extended to generate 1-10 mW of THz output in a compact setup.